1. Field of the Invention
This invention concerns a modulated spectroscopic ellipsometer.
2. Background Information
EIlipsometry is a non-destructive measuring technique allowing optical characterization of a sample having a specular or almost specular surface.
Ellipsometry can be performed in situ, and may therefore be used to study mechanisms of thin film growth and interface formation, and the control of the development processes of these films and interfaces. Ellipsometry is used, for example, to study and control semiconductor fabrication.
Ellipsometric measurements can be taken at a fixed or variable wavelength (spectroscopic ellipsometry). They form part of the group of specular measurement techniques, which includes for example reflectance, modulated reflectance (electroreflectance, photoreflectance), anisotropic reflectance (spectroscopic differential reflectance), absorption and transmission measurements, with which they can be associated. An ellipsometric measurement is taken by illuminating the surface of a sample with a light beam and comparing the polarization state of the reflected or transmitted light beam r to that of the incident beam i. The polarization vector E is generally represented by its projections E.sub.s and E.sub.p, respectively perpendicular and parallel to the incidence plane; E.sub.p and E.sub.s are complex amplitudes.
In the field of ellipsometry, the ratio (E.sub.p /E.sub.s).sup.r / (E.sub.p /E.sub.s).sup.i denoting the changes in the polarization state produced by the surface studied is generally represented in the following form: EQU tg.psi..exp(i.DELTA.)=(E.sub.p /E.sub.s).sup.r /(E.sub.p /E.sub.s).sup.i
The object of ellipsometry is therefore to measure the parameters .psi. and .DELTA. for a given surface.
The wavelength domain and the various conditions for taking the measurement, such as the polarization modulation frequency, and in turn the apparatus and ellipsometers to be used, are determined from the materials or phenomena to be studied.
French patent FR-A-2.602.338 describes a phase-modulated, Fourier-transform, infrared ellipsometer.
Physicists have recently discovered the interest of characterizing films and stacked films or interfaces by means of ellipsometric measurements taken in the presence or absence of external excitation and in comparing the measurements obtained in the presence or absence of excitation.
This excitation can, for example, be thermal, optical, electrical or magnetic.
For more details on this subject, the reader is invited to refer to the article "Modulated Ellipsometric Measurements and Transfer-matrix Calculation of the Field-dependent Dielectric Function of a Multiple Quantum Well" by J.-Th. Zettler (The American Physical Society--Phys. Rev. B46, 15955-1992) and to the article "Photoellipsometry Determination of Surface Fermi Level in GaAs (100)" extract from the Journal of Vacuum Science Technology A 11(4), July/August 1993, by Yi-Ming Xiong.